Thermal protection for power lines is accomplished by monitoring the temperature of the wire conductor and generating a trip signal which opens a circuit breaker when the conductor temperature becomes greater than the maximum allowed temperature for the conductor. The temperature of the conductor is not measured directly, but is obtained by using a first order thermal model for the conductor. This involves using a heat balance equation (heat input minus heat losses) of a 1000 foot section of conductor.
The 1000 foot section of the line is assumed to have the highest temperature and to be exposed to the maximum solar radiation. The heat input portion in the equation is mainly due to heat dissipated in the conductor resistance and the solar heat gain. The heat loss portion is due primarily to convection and radiation. In general, the equations for establishing such a thermal model are well known, and the procedures for obtaining the temperatures of overhead conductors using a thermal model are set forth in IEEE Standard 738-1193, titled “IEEE Standard for Calculating the Current-Temperature Relationship of Bare Overhead Conductors”.
A power line thermal model has been used previously in protective relays, although the thermal model equations are preestablished and the parameters thereof are determined by the manufacturer. The user has no control over the thermal model or its operation. It is an automatic process. This can lead to inaccurate results in some cases. Accordingly, it is desirable that the end user of a protective relay have some capability to itself establish the thermal model parameters for a more accurate and fast determination of power line temperature.